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Keeping a motor at stall torque

Hi,

I wanted to bring up a question that I’ve been a little confused about: that is, if I have a motor and it meets a resistance it cannot overcome, how do I keep it at stall torque without burning it out?

For example, I have a power supply capable of producing more than enough amps to burn out a motor+motor driver on one end. I have a my stalled motor on the other end. What goes in between so that i deliver the maximum amount of current without having to worry about burning my motor/electronics?

Mike

Hello, Mike.

To prevent a stalled motor from damaging a motor driver or controller, you should make sure that the driver’s continuous current rating is higher than the motor’s stall current at the voltage you are using. However, most motors are not designed to be stalled for extended periods at their rated voltage and current, and doing so will probably drastically shorten the lifespan of the motor.

If you do need a motor to exert a static torque for long periods of time, one solution might be to use a controller that can actively limit the current through the motor to less than its full stall current (such as our jrk motor controllers). You will not get the motor’s full rated stall torque this way, so you would have to choose a more powerful motor and use a lower current limit that still allows it to exert enough torque.

- Kevin

So I should use some factor of safety from the stalled current rating so that the motor doesn’t overheat? What should I set the motor driver current limit to be w.r.t. the stalled current rating of the motor? And then would this be the minimum continuous current setting of the motor driver?

for the current sensing and limiting, is that done through something like a microcontroller or can get the motor driver to do it? I ask this because if the microcontroller program ever fails or hangs then the current sensing also will not work properly.

In general, the lower you can keep the current going through the motor relative to its stall current (the closer it is to the no-load current), the longer you can expect the motor to last. A good starting point might be to try to limit the current to no more than 20-30% of the stall current.

Depending on how you set up your system, you can either have your main microcontroller handle the current limiting or let a suitable motor controller take care of it. The jrk motor controllers I linked to have their own built-in current sensors and microcontrollers, so they can automatically limit the current to a value you specify (and they should continue to do so even if your main controller hangs or stops working).

- Kevin

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If I have a motor that runs at 4.5A under max load but has a stall current of 15A, can I still use the pololu G2 24v13 Pololu G2 High-Power Motor Driver 24v13, or must I use a different one of your motor drivers that are rated for a current over 15A?

Hello, zxq.

We generally recommend choosing a motor driver that can handle the stall current of your motor continuously, but the 24v13 G2 High-Power Motor Driver should generally be fine for that motor, especially if you are only pulling 4.5A max.

Please note that driving the motor in certain ways can cause very large spikes of current that can be hard to measure due to how fast they occur. For example, switching directions at full speed can draw upwards of twice the stall current. Similarly, commanding the motor to run at full speed from a stopped state can cause a current spike as high as the stall current, although I generally expect the 24v13 G2 High-Power Motor Driver to be able to handle a 15A spike like that without problems.

Brandon

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